player vs understudy

player and understudy both are nouns.

Word	Noun	Adjective	Verb	Adverb
player	Yes	No	No	No
understudy	Yes	No	Yes	No

As nouns, understudy is a hyponym of player; that is, understudy is a word with a more specific, narrower meaning than player:

player (noun)	understudy (noun)
a theatrical performer	an actor able to replace a regular performer when required
a person who participates in or is skilled at some game
someone who plays a music as a profession
a person who pursues a number of different social and sexual partners simultaneously
someone who takes part in an activity

player (verb)	understudy (verb)
	be an understudy or alternate for a role